Lasers have been adapted to a wide variety of applications, depending at least in part upon the power of the laser. Frequently, amplification stages are added to lasers to increase their power output, but this has deleterious effects, as known in the art. For those applications requiring a high power, single mode coherent light source, present-day single mode semi-conductor lasers cannot be scaled up to higher power levels without losing their single mode coherence. The designs presently available are all lower power devices, and hence there is a real need for a way to amplify these devices without sacrificing coherence, etc.
One approach to solving this problem is to combine several lasers in a coherent manner. It is known how to phase-lock a semi-conductor laser to a master oscillator. However, once the several lasers are in phase-lock with a single master oscillator, it is not known how to adjust the phases of the several devices so that a plane wave front is generated and the radiation is directed in only one direction.
Another approach to solving the problem has been described in a paper by D. Scifres, R. D. Burnham, W. Streifer, and M. Bernstein, and published at Appl. Phys. Lett. 41, 614 (1982). That device includes a single bar, semi-conductor laser array in which the authors claim that the device constitutes a phased array. However, the data supplied in the paper is sketchy, and there is reason to believe that the disclosed device did not operate in a single mode. Still another alternative approach is to use the spatial interference between a master array element and each other array element as the source of feedback phase information. That approach suffers two drawbacks--it is sensitive to mechanical positioning, and no practical means of implementing it is known.
The method and apparatus of the present invention produces a high power, single mode coherent light source with one or more stages of amplification, and which maintains its in-phase relationship through a number of feedback loops one less than the number of amplified optical signals. The approach utilized is to separately, individually modulate a plurality n of optical signals from a single optical source before each signal is then amplified, the modulation including shifting the phase of the individual optical signals to bring the optical signals into phase. The combined output of the amplified beam is detected, and an electrical signal corresponding thereto produced which is then filtered into a number (n-1) of frequency components for electrically producing a number (n-1) of phase correction functions. These phase correction functions are then combined in various combinations to produce a plurality (n) of phase shift signals to drive the separate individual modulating phase shifters which are associated with each optical signal. The relationship between the phase correction functions and the phase shift signals can be defined by a transformation matrix of n-1.times.n size, and if that transformation maxtrix has a rank of n-1 or rows which are linearly independent, then the system of the present invention will function to maintain the separate optical signals in phase. Further, if the row vectors of the transformation matrix are mutually orthogonal, then each feedback loop exhibits the same gain and stability which further enhances the operation of the system.
It can be shown using the theory of linear transformations that it is only necessary for the system to include n-1 feedback loops each having a separate frequency to produce a number n of phase correction signals for modulating a number n of optical signals into phase. This technique provides several advantages in that the entire output beam is detected and used to produce the single feedback signal. This ensures the complete capture of error information and processing by the feedback loops. Additionally, a minimum number of feedback loops are used which results in a cost savings and reduces the complexity of the system. Furthermore, this basic principle can be used on any number of optical signals, the number of feedback loops always being one less than the number of processed optical signals. Therefore, as the power of an optical signal is generally limited by the power of the amplifier used, several stages of amplification can be used with a single optical source to geometrically increase the power of the beam and yet maintain all the advantages inherent with a single optical source such as coherence, etc. The apparatus and method will now be described in more detail.
The system utilizes a single optical source which is split into a plurality of optical signals by fiber conductors, each signal being processed by a modulating phase shifter and then by an individual amplifier to produce a high power, single mode coherent beam. The combined output of the amplified light souce is detected and used to produce a single electrical feedback signal. This feedback signal is filtered into a number of frequency based components, the number of frequency components being one less than the number of separately amplified optical signals, one frequency component being used in each of n-1 feedback loops. The component is then compared with an independently generated signal of the same frequency, the phase difference detected, and then combined with the independently generated signal of the given frequency to produce a phase correction function. Balanced output amplifiers are used to generate positive and negative phase correction functions, these positive and negative functions being combined to produce a number of phase shift signals.
The number of phase shift signals produced is equal to the number of individually amplified optical signals, and each phase shift signal is fed to a driver which in turn drives the modulating phase shifter to modulate the individual optical signal and shift its phase to bring it into phase with the other optical signals. It is noted that the relationship between the phase correction functions and the phase shift signals are defined by the electrical connections therebetween as a transformation matrix which has the property of a rank of n-1 or rows which are linearly independent to assure that the combined output is in phase. If connections equivalent to a transformation matrix having row vectors which are mutually orthogonal are used to combine the phase correction functions into phase shift signals, then each feedback loop should experience the same gain and stability which should further enhance the operation of the system. Of course, additional stages of amplification can be used to produce an even greater number of individual optical signals, it only being required that a modulating phase shifter be associated with each optical signal before it is amplified by the final stage of amplification.
By using the apparatus and method of the present invention, a single mode high power coherent light source can be produced.